Controlled adhesion conductor

ABSTRACT

Compositions useful for printing controllable adhesion conductive patterns on a printed circuit board include finely divided copper powder, a screening agent and a binder. The binder is designed to provide controllable adhesion of the copper layer formed after sintering to the substrate so that the layer can lift off the substrate in response to thermal stress. Additionally, the binder serves to promote good cohesion between the copper particles to provide good mechanical strength to the copper layer so that it can tolerate lift off without fracture.

This is a division of U.S. application Ser. No. 07/297275 filed on Jan.17, 1989, which is a continuation-in-part of U.S. application Ser. No.07/232,744 filed on Aug. 16, 1988.

CROSS REFERENCE

This is a continuation-in-part of our patent application, Ser. No.232,744, filed Aug. 16, 1988, and having a common assignee andinventorship and is related to our copending patent application, Ser.No. 232,743, filed Aug. 16, 1988, and having a common assignee andinventorship, and is also related to two other patent applications, Ser.Nos. 297,212 and 297,201, which are being filed concurrently with thepresent patent application and which have a common assignee andinventorship.

FIELD OF THE INVENTION

This invention relates to compositions of matter that are useful informing conductive patterns on an insulating substrate. One suchsubstrate is a circuit board having printed conductors which connect anelectrical module to the circuit board to form a printed circuit boardassembly.

BACKGROUND OF THE INVENTION

As used herein, the term "electrical module" is intended to include anycomponent, such as a semiconductor chip, a light emitting or detectingdevice, an emitting or detecting device of magnetic nature, a magneticstorage device, a capacitor, an inductor, a resistor, a crystal, a coil,a varactor, a thermistor, a resonator, transformers, and/or a connectorthat may be connected in an electrical circuit, an electro-opticalcircuit, an optical configuration, an electro-magnetic circuit and/or amagnetic configuration. The term "printed circuit board" is intended toencompass a support member that may be rigid or flexible, laminated ornot, of any suitable composition on which a printed pattern ofelectrical conductors is provided. The term "printed pattern" isintended to encompass a pattern on a substrate formed by any suitableprocess, including, but not limited to, silk screen printing,photolithography, evaporation, plating, stenciling, ink writing, plasmadeposition, sputtering, adhesives, or thin film or thick filmtechniques.

A printed circuit board having a chip or other member attached theretoshall be denoted as a printed circuit board assembly. It will beconvenient to generally discuss the invention with particular referenceto a printed circuit board assembly including a semiconductor chip ofthe surface mount or flip-chip type.

Such a chip typically has formed thereon an integrated circuitcontaining a number of electrical contacts (terminals) on one surface.The contacts are typically soldered or otherwise electrically connectedto the conductors of the printed circuit board.

In the past, much effort has been focused on the use of conductors thatare firmly adherent to the printed circuit board to minimize possibleseparation during use. Additionally, effort has also focused onmaintaining the integrity of the conductor-chip interface to preventfailure at that connection. This arrangement usually operatessatisfactorily. However, there are hostile environments, usually of hightemperature and thermal excursions, where, during use, severe stressescause the chip to separate from the printed circuit at or near thepoints of connections, i.e., the solder joints. Such stresses can occur,for example, when the printed circuit board is used in an automobile.

It is desirable to have conductors of compositions which permit aprinted circuit board assembly in which the electrical modules remainreliably connected to the printed conductors in hostile environments.

SUMMARY OF THE INVENTION

In a copending application, Ser. No. 232,743, filed Aug. 16, 1988, andhaving a common assignee, and in a continuation-in-part patentapplication thereof which is being filed concurrently with the presentpatent application, there is described and claimed a printed circuitboard assembly which employs controllably adherent conductors that areconnected at one end to a contact area of an electrical module and atthe other end to a contact portion of a firmly adherent conductor on thecircuit board (substrate). The term "controllably adherent conductor" isused to describe a conductor which adheres to the circuit board duringthe manufacturing process, but which is free when being used, to move(slide) along and/or lift off from the circuit board in response tostresses that may arise, for example, from thermal effects, to relievesuch stresses.

This represents an approach that is the antithesis of the conventionalapproach mentioned above that has focused on maintaining the conductorsfirmly adherent to the circuit board. However, this approach doesrequire a conductive layer for use as the controllably adherentconductor that has a degree of cohesive strength so that it does notfracture or become mechanically weak or electrically defective whenseparated from the circuit board.

The present invention is directed to compositions useful for formingsuch controllably adherent conductors.

In particular, the compositions in accordance with the invention includea metal powder, typically copper, as the predominant or major fraction.The compositions also include an organic vehicle as a screening agent,typically a mixture of a solvent and a resin. Additionally, thecomposition includes, typically as the smallest fraction, an appropriatebinder to provide good cohesion between the metal particles and toimpart the desired controllable adhesion to the circuit board substrate.Typically, the binder is a few percent by weight of bismuth oxide or amixture of bismuth oxide and cuprous oxide. Additionally, various glassfrits may optionally be included in the binder as will be described inmore detail below.

The invention will be better understood from the following more detaileddescription taken with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a portion of a printed circuit boardassembly whose preparation involves a composition in accordance with oneembodiment of the invention;

FIG. 2 is a top view of a portion of the assembly shown in FIG. 1;

FIG. 3 is a top view of a modification of controlled adhesion conductorsof the assembly of FIG. 1;

FIG. 4 is a top view of another printed circuit board assembly;

FIG. 5 is a top view of another printed circuit board assembly,

FIG. 6 is a top view of another printed circuit board assembly;

FIG. 7 is a top view of another printed circuit board assembly;

FIG. 8 is a flow chart of a typical process in accordance with theinvention which is used to form the assemblies shown in FIGS. 1, 4, 5,6, 7 and 9;

FIG. 9 is a cross-sectional view of a portion of a printed circuit boardassembly whose preparation involves a composition in accordance with theinvention; and

FIG. 10 is a graphical plot of the parameters of a furnace useful informing the conductors on the printed circuit board assemblies of theabove describe figures;

FIG. 11 is a cross-sectional view of a portion of a printed circuitboard assembly shown in the previously mentioned patent applicationswhich are being filed concurrently with the present patent application;and

FIG. 12 is a cross-sectional view of a portion of a printed circuitboard assembly shown in the previously mentioned patent applicationswhich are being filed concurrently with the present patent application.

It is to be noted that the draiings are not to scale. Some backgroundlines are not shown and some portions may omit cross hatching to makethe drawings easier to understand.

DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is shown a cross-sectional view of aportion of a printed circuit board assembly 10 whose preparation uses acomposition in accordance with one embodiment of the invention. Theassembly 10 comprises an electrical module 20 and a support member(circuit board, substrate) 40 having an insulative top surface 42 onwhich are printed (formed) firmly adherent conductors 50 andcontrollably adherent conductors 60. In a typical embodiment the module20 comprises an integrated circuit formed in a silicon chip of the flipchip or surface mount type, suitably covered by an insulating material(not shown) and having a plurality of external contacts 30 (e.g., silverbumps) on a lower surface 34 thereof. These contacts 30 are connected toelectrodes (not shown) of various circuit elements (not shown) of themodule 20. The electrical module 20 may take various other forms aspreviously mentioned. It is to be noted that, for illustrative purposesonly, one module 20 is shown. Typically, a printed circuit boardassembly contains a plurality of modules 20 of different types.

Substrate 40 is of a suitable insulating material, such as alumina (Al₂O₃). The conductive pattern formed by the conductors 50 and 60 and themodule 20 form an electrical circuit of the printed circuit boardassembly 10. The firmly adherent conductors 50 may be conventional. Theyare typically formed by depositing a patterned layer of a suitable inkor paste that, after firing, provides a firmly adherent conductivepattern that is compatible for use with a pattern of controllablyadherent conductors 60. Such a firmly adherent conductor may includecopper, nickel, silver, palladium, platinum and gold and/or alloysthereof and/or other suitable metals and/or compounds to make itconductive.

The module 20 is secured to the highly adherent conductors 50 by way ofcontrollably adherent conductors 60. Each contact 30 of module 20 istypically connected to one end 60a of a controllably adherent conductor60 by means of a bond, such as a solder joint 32. The nature of theseconductors 60 will be discussed more fully below. The bond could beformed in various other ways, including use of a conductive adhesiveepoxy or other conductive adhesive material, or the bond could be aweld. The other end 60b of the controllably adherent conductor 60 isbonded or fused in any suitable fashion to a contact portion 50a,typically formed by an end of a firmly adherent conductor 50. All butthe latter end 60b of each conductor 60 is controllably adherent to thesurface 42 so that it can flex and lift off the surface 42 to relievestresses on solder joints 32. Moreover, because the contacts 30 of themodule 20 are bound only to the ends 60a of conductors 60, and themodule 20 is not itself bonded to surface 42, the module 20 can alsomove along or lift off the surface 42 of the substrate 40 to helprelieve stresses at the joints 32. As a result, pulling apart forces aresignificantly reduced at each solder joint 32. The stresses that occurmay produce either tensile or compressive stresses that aresubstantially completely absorbed due to the flexible nature in whichmodule 20 is mounted to provide a substantially stress-free andfatigue-free system.

Moreover, as will be discussed more fully later, because thecontrollably adherent conductors 60 are free to move, it is feasible todesign the lengths of the conductors 60 so that their changes in lengthrelative to the changes in the lengths of the relevant portion of thealumina substrate 40 and of the electrical module 20 balance over anoperating temperature range to further relieve stresses in the solderjoints 32.

Referring now to FIG. 8, there is shown a flow chart of a typicalprocess for preparing the assembly 10 shown in FIG. 1. As indicated, aceramic printed circuit board, typically of alumina, is used as thesubstrate 40.

Firmly adherent conductors 50 are then deposited on the substrate 40.This is done in conventional fashion and generally involves first screenprinting a suitably patterned layer and then drying and firing thelayer. The layer as printed, is of an ink or paste that will form afirmly adhering conductive layer. In a preferred embodiment the layerdeposited is of a material commercially available from DupontElectronics and known as Dupont 9161. It basically comprises conductivecopper particles, adhesion promoters and a screening agent. Suitableother materials are available from other sources. After printing, thedrying and firing is done in the manner prescribed by the supplier tosinter the copper particles together into a conductive layer 50 whichfirmly adheres to the substrate 40.

Referring now to FIG. 10, there is shown a graphical plot of theparameters of a furnace (not shown) that can be used for the firingprocess. In the plot, the y axis shows temperature in degrees C ofsuccessive zones of the furnace and the x axis shows elapsed time inminutes as a workpiece (e.g., a circuit board 40 with ink patterns whichwill become conductors 50 and/or 60) moves along a belt (not shown)through the various temperature zones existing within the furnace. Theterms "ENTRANCE" and "EXIT" shown in FIG. 10 indicate the workpieceentering and exiting, respectively, the furnace. As is seen, theworkpiece takes slightly more than twenty minutes to reach the maximumtemperature of about 900° C. during which time the organic vehicle inthe original paste or ink is being removed. It then takes about tenminutes to pass through the furnace zone at this peak temperature duringwhich the copper particles are sintered into a conductive layer.Finally, it is allowed to cool gradually to room temperature over abouta twenty minute period to reduce thermal shock. Of course, the optimumparameters are usually a function of the particular paste or ink beingused.

Next the controllably adherent conductors 60 are similarly formed byscreen printing, drying and firing a layer of suitable composition.

To avoid the need for separate furnaces, it is advantageous to choosecompositions for use in forming both the firmly adherent conductors 50and the controllably adherent conductors 60 that can be processed in thesame way. When this is done, the drying and firing of the compositionused to form conductors 60 are carried out in the same manner asdescribed previously for the conductors 50 as discussed with referenceto FIG. 10. The conductors 60 are formed to extend between the firmlyadherent conductors 50 and where the contacts 30 of the modules 20 areto be positioned.

Compositions in accordance with the invention are used to formconductors 60 to provide the desired controllable adherent properties inaddition to being compatible with use with conductors 50.

Several compositions are presently of particular interest for use toform controllably adherent conductors of the kind described. Thesecompositions have the advantage that they have the desired compatibilitywith conductors 50.

The first composition, which is preferred for some applications,includes by weight about 88.5 percent of a conductor, about 10 percentof a suitable screening agent, and about 1.5 percent of a binder. Theconductor is finely divided copper powder with an average particle sizeof between 1 and 2 microns. The specific screening agent is a mixture ofabout 90 percent by weight of a solvent, typically TEXANOL™ (believed tobe 2,2,4 Trimethyl 1,3 Pentanedio Monoisobutyrate), and the remainder ofa resin, typically ethyl cellulose grade N-50, both of which areavailable commercially. The binder consists essentially of bismuth oxide(Bi₂ O₃) which serves to promote both adhesion of the layer 60 to thesubstrate 40 during processing and cohesion of the copper particlesafter firing.

Too large an amount of the binder undesirably increases adhesion to thesubstrate 40 and makes it difficult for the controllably adherentconductors 60 to lift off the surface 42 of the substrate 40 toaccommodate mechanical stresses. Too little of the binder results innonadherence immediately after firing, preventing any subsequentprocessing. Typically the binder should be at least 0.05 percent and nomore than 5.0 percent of the composition, although if the binder ischosen to be one largely lost during firing, or is not a very effectiveadhesion promoter, larger amounts initially may be feasible. Theappropriate limits can be expected to be dependent on thecharacteristics of the metal powder employed.

The screening agent should be chosen to facilitate the screen printing,and various other agents should be suitable. Moreover, the fraction ofthe solvent in the screening agent typically is between 85 and 98percent.

A second composition, which is also found particularly useful, differsonly in the composition of the binder which consists essentially ofequal parts by weight of bismuth oxide and cuprous oxide, milledtogether. Here, the cuprous oxide appears to serve primarily to promoteadhesion.

Another composition that proved useful included by weight about 89percent powdered copper, about one percent a frit as the binder, andabout 10 percent of the screening agent described above in which thefrit included by weight about 70.0 percent bismuth oxide (B₂ O₃), 15.6percent lead oxide (PbO), 4.5 percent lead fluoride (PbF₂), 6.6 percentsilicon dioxide (SiO₂), 0.6 percent aluminum oxide (Al₂ O₃), and 2.7percent boron oxide (B₂ O₃).

The components of this frit by weight could be varied with the Bi₂ O₃between 0-73 percent, the PbO between 15-60 percent, the PbF₂ between4-12 percent, the SiO₂ between 5-25 percent and the between 0.5-2.5percent and the B₂ O₃ between 2-10 percent. This frit can be also viewedas a mixture of bismuth oxide (0-70 percent) and the remainder a frit ofthe following composition by weight, 50-65 percent PbO, 5-15 percentPbF₂, 20-30 percent SiO₂, 0-5 percent Al₂ O₃, and 5-15 percent B₂ O₃with a typical composition of this frit by weight 55.4 percent PbO, 9.4percent PbF₂, 23.5 percent SiO₂, 2.1 percent Al₂ O₃ and 9.6 percent B₂O₃.

Two other compositions that proved successful included by weight 89.25and 89.5 percent copper powder, respectively, 10 percent of thescreening agent described earlier and the balance a frit that includedby weight 60-75 percent PbO, 10-25 percent SiO₂, 5-30 percent B₂ O₃ and0-10 percent Al₂ O₃. One embodiment which proved successful included aglass frit approximately by weight 66.6 percent PbO, 22.4 percent SiO₂,8.7 percent B₂ O₃ and 2.3 percent Al₂ O₃.

These examples demonstrate the wide range of mixtures that can be usedas the binder in the compositions to achieve the controllably adherentconductor needed.

In general, the compositions especially useful for forming controllablyadherent conductors 60 comprise between 75 and 92 percent finely dividedcopper powder, with an average particle size of between about 0.5 and5.0 microns, 7.5 to 20 percent of a screening agent, and the remainderessentially of a binder to promote adequate cohesion and controllableadhesion of the kind described. As a preferred range, the copper has anaverage particle size between 1 and 2 microns and comprises between 85and 90 percent of the mixture.

A variety of other compositions should be suitable including some thatuse other kinds of conductive particles, such as silver, palladium,nickel, platinum and gold and/or alloys thereof. Moreover, in someinstances, it may prove advantageous to include a surfactant to improvewetting. The composition should result in controllably adherentconductive layers having characteristics compatible with those of thefirmly adherent conductive layers.

Next, optionally there may be printed, dried and fired other layers thatserve special roles, such as thick film resistors. In some instances, inorder to provide different values of thick film resistors, two or moreresistive compositions are included that require separate processing.This step may be repeated as needed to deposit all such thick filmresistors.

Finally, as indicated, the electrical module 20 of the assembly 10 iselectrically connected to the controllably adherent conductors 60.

Typically, this first includes printing solder dots on the ends 60a ofconductors 60 where such ends 60a are to be attached to the contacts 30of the module 20. This can be done in conventional fashion.Advantageously, the solder paste used to form the dots comprises a 25/75tin-lead solder. After the solder paste is deposited, the electricalmodule 20 is appropriately positioned to have its contacts 30 alignedwith the solder dots. Then the assembly 10 is heated to a temperature tomelt and reflow the solder and then cooled to solidify resulting solderjoints 32 between the contacts 30 and the ends 60a of conductors 60.

It should be apparent that it would be feasible, if desired, to print orform the controllably adherent conductors 60 before printing or formingthe firmly adherent conductors 50.

In some instances it may be feasible to use a single furnace operationto form both patterns of conductors simultaneously.

When the assembly 10 thus prepared is placed in operation and subjectedto sufficiently high stresses, these stresses are absorbed by theconductors 60 which lose their adhesion to the substrate 40 and then arefree to move along and/or lift off the surface 42 of the substrate 40.

In an embodiment of assembly 10 in accordance with the invention: thecontacts 30 of module 20 are made of silver and each has a height in therange of 0.0007" to 0.003" and a diameter of about 0.006"; the module 20is square in shape and has dimensions in the range of 0.180" to 0.3";and the solder joints 32 are formed using a 25/75 tin lead solder; eachof conductors 60 has a width of 0.005" to 0.01", a length of 0.04" to0.150" and a thickness of 0.0003" to 0.0008". Various other parameters,such as the thickness of conductors 60 in the range of 0.0003 to 0.001inches, are, of course, feasible.

The dimensions of the conductors 60 advantageously are designed toprovide optimal performance of their function with a minimal amount ofconductor material. Various conductor configurations are feasible andtypical configurations are described below.

Referring again to FIG. 1, in one presently preferred design the thermalexpansion rate (change in length/unit change in temperature) ofdimension A to D along the conductors 60 and the module (chip) 20 isequal to that of the dimension of E to F along the substrate 40. Therate of expansion is determined by the thermal expansion coefficient ofthe type of material and the length of material that is of concern.Thus, if the rate of expansion of distance A to B, B to C and C to D areadded, they become that of A to D, which should be equal to the rate ofexpansion of E to F. For a given assembly 10, the dimension B to C isfixed by the chip 20 design. However, the dimensions A to B and C to Dcan be made of any desirable length. By adjusting the length of A to Band C to D (normally these would be equal), the sums of the rates of allthree components can be made equal to that of E to F. One assembly 10,which was built on an alumina substrate 40, had a length (A to B)=(C toD)=0.44 inches and used a module 20 having a square dimension of 0.284"(B to C).

The controllably adherent conductors may be formed in various shapes asdesired, since the shape, while important, is not critical.

Referring now to FIG. 2, there is shown a top view of a portion ofassembly 10 which shows the controllably adherent conductors 60 asstraight.

Referring now to FIG. 3, there is shown a top view of controllablyadherent conductors 603 which have an S-shaped bend to reduce stress atthe joint between a conductor 603 and a firmly adherent conductor 503.Conductors 503 and 603 are the same kind of conductors as conductors 50and 60, of FIG. 1 except for the bend in each conductor 603.

Referring now to FIG. 4, there is shown a top view of a printed circuitboard assembly 104. Assembly 104 is similar to assembly 10 of FIG. 1 andall corresponding parts thereof have the same reference number with a"4" added thereafter. One end of each of firmly adherent conductors 504mechanically and electrically contacts a bonding (contact) pad 52 whichfirmly adheres to surface 424 of substrate 404. Controllably adherentconductors 604 are straight. Each of conductors 604 is shown having twoparallel fingers. This configuration is useful for testing, but a singlefinger conductor 604 is typically preferable. It is to be noted that theconductors 504 can be eliminated with conductors 604 then beingconnected directly to pads 52. Still further, pads 52 can be modified inshape and size and brought out to an edge of substrate 404 to serve asthe male portion of a connector assembly (not shown) that is adapted toplug into a female portion (not shown) of the connector assembly.

Referring now to FIG. 5, there is shown a top view of a printed circuitboard assembly 105. Assembly 105 is similar to assembly 10 in FIG. 1 andall corresponding parts thereof have the same reference number with a"5" added thereafter. Assembly 105 is also similar to assembly 104 ofFIG. 4 except that controllably adherent conductors 605 are S-shaped andare connected to firmly adherent conductors 505 which are connected tobonding (contact) pads 525. A module 205 to which conductors 605 areconnected is not shown.

Referring now to FIG. 6, there is shown a top view of a printed circuitboard assembly 106. Assembly 106 is similar to assembly 10 of FIG. 1 andall corresponding parts thereof have the same reference number with a"6" added thereafter. Assembly 106 is also similar to assembly 105 ofFIG. 5 except that the controllably adherent conductors 606 are angledwith respect to the module (chip) 206 (not shown) they contact. Printedcircuit board assembly 106 allows the chip to rotate so as to relievestress where the chip is soldered to conductors 606.

Referring now to FIG. 7, there is shown a top view of a printed circuitassembly 107. Assembly 107 is very similar to assembly 10 of FIG. 1 andto assembly 104 of FIG. 4 except the corresponding parts thereof have a"7" added to the last digit of the reference number. In this case, thereis no provision for the rotation of the module (chip) 207 (not shown) toaccommodate movement in the positions of its contacts 307 (not shown)and the joints 327 (not shown). Instead, the length of the pairs ofconductors 607 are chosen so that over a preselected operatingtemperature range the changes in position of their first ends are suchas to compensate for the changes in position of the module contacts 307(not shown). As a result, stresses on the solder joints 327 aresignificantly limited and the reliability of assembly 107 issignificantly increased.

It should be apparent that the invention is largely independent of thespecific nature of the electrical module connected into the printedcircuit. It could be simply any circuit component as listed above thathas contacts on one surface by which the circuit component is to beconnected. It may also be a connector whose various pins are to beconnected to ends of the controllably adherent conductors.

Additionally, as was mentioned earlier, the term "printed circuit board"can encompass a wide variety of forms since the nature of the substrateon which the controllably adherent conductors are deposited need only becompatible with the desired controllable adhesion of the conductivelayer deposited.

In particular, the substrate or support member may even be asemiconductor wafer on which are deposited controllably adherentconductors for connecting components therein. Additionally, thesubstrate or support member may be a first semiconductor chip on top ofwhich is connected a second semiconductor chip by way of controllablyadherent conductors which are free to move on and/or away from a surfaceof the first chip.

Moreover, the invention may be feasible with superconductive circuits inwhich either the controllably adherent or firmly adherent conductors aresuperconductors.

Similarly, it should be apparent that the specific nature of the circuitboard that serve as the substrate or support member is not critical. Inparticular, the circuit board need not be rigid but may be of a flexiblekind. Also, it need not be ceramic but may be of any suitablecomposition that is compatible with controlled adhesion.

In some instances, it may prove desirable to provide a resilient coatingover the surface of the electrical module including its contacts, tocushion any movement of the module. In other instances, particularlywhen a module is of relatively large mass, it may be desirable torestrain its movement.

Referring now to FIG. 9, there is shown a cross-sectional view of aportion of a printed circuit board assembly 109 whose preparation uses acomposition in accordance with the invention. Assembly 109 is verysimilar to assembly 10 of FIG. 1 and the corresponding parts thereofhave a "9" added to the last digit of the reference number. Assembly 109has an electrical module 209 which has a plurality of external contacts(bumps) 309 and has an essentially centrally located contact (bump) 90which is soldered via a solder joint 329 to a firmly adherent conductivecontact region 529 on the surface 429 of the support member 409. Thisresults in the center portion of module 209 being essentially fixed tothe surface 429 of support member 409. Portions of module 209 other thanthe center portion thereof are free to move to relieve stress as areconductors 609.

A via 92 (shown in dashed lines) can be formed through support member409 and a conductor 94 can be passed therethrough to an opposite surface96 of support member 409. A firmly adherent conductor 98 (shown indashed lines) is attached to one end of conductor 94, which has a secondend attached to contact region 529. Thus, support member 409 can be usedas a double (two) sided printed circuit board. A central portion ofsupport member 409, the portion shown as having contact 90, could havecontact 90 deleted and could be instead adhesively attached to thesurface 429 by a heat conducting and electrically non-conducting epoxyor other material or compound (all not shown) to help conduct heat awayfrom module 209. This added heat dissipation mechanism allows for theuse of larger chips with associated higher power dissipation than mightbe otherwise possible. In some instances, the epoxy could beelectrically conductive and could be used to bring electrical signalsand/or power to the module 209. A plurality of contacts, each likecontact 90, could be substituted for contact 90. These substitutedcontacts would be attached to central portions of module 90 and wouldserve to enhance the mechanical integrity of assembly 109 duringvibration of same. The substituted contacts could also serve to bringelectrical signals and/or power to module 209.

It should also be apparent that the printed circuit board may supportone or more electrical modules with some free to move on the circuitboard in the manner discussed and with others that are fixed in place inconventional fashion.

Referring now to FIG. 11, there is a cross-sectional view of a portionof a printed circuit board assembly 1011 in accordance with anembodiment of the invention. Assembly 1011 is essentially the same asassembly 10 of FIG. 1 except that it comprises a second electricalmodule 20a that is attached (soldered) via contacts (e.g., silver bumps)30a by soldered joints 32a to portions 50b of firmly adherent conductors50. Such attachment is typically used when module 20a is sufficientlyheavy such that if it were attached to controllably adherent conductors60, it could endanger the integrity of the controllably adherentconductors 60 and cause them to crack and/or break. The components ofassembly 1011, which are very similar or identical to those of assembly10 of FIG. 1, have the same reference number. Modules 20a advantageouslyare fabricated from materials which have a coefficient of thermalexpansion which is close to that of support member 40 and thereforesolder joints 30a will experience little stress as a result of thermalchanges. Typically, thick or thin film resistors printed on a circuitboard would be connected to firmly adherent conductors as would a chipthat is to be wire bonded to the circuit board.

If one composition is used to form all conductors, differences inadhesion to the circuit board can be achieved by modifying portions ofsubstrate on a microscopic scale to alter adhesion, whereas otherportions of the circuit board are left in the normal state. Anothertechnique which uses one composition to form both firmly adherent andcontrollably adherent conductors on a common circuit board is describedbelow.

Referring now to FIG. 12, there is shown a cross-sectional view of aportion of a printed circuit board assembly 1012. The assembly 1012comprises electrical modules 2012 and 2014 and a support member (circuitboard, substrate) 4012 having an insulative top surface 4212 on whichare printed (formed) conductors 1014 having portions 1014a which areseparated from surface 4212 by an inhibitor layer 1016 and havingportions 1014b which make direct contact with surface 4212. The portions1014a of conductors 1014 where the inhibitor layer 1016 liesintermediate between surface 4212 and the conductive layer 1014 arecontrollably adherent. The portions 1014b of conductors 1014 where theconductor 1014 is in intimate contact with the surface 4212 are firmlyadherent.

The electrical module 2012 has contacts 3012 thereof (which are attachedto a bottom surface 3412 of module 2012) bonded, typically by solderjoints 3212, to the portions 1014a of conductors 1014. As a result, theportions 1014a of conductors 1014 are controllably adherent and are ableto move along and/or lift off surface 4212 of the circuit board 4012 asneeded to relieve mechanical stresses on the solder joints 3212.

The electrical module 2014 has contacts 31 thereof (which are attachedto a bottom surface 33 thereof) bonded, typically by solder joints 35 tofirmly adherent portions 1014b of conductors 1014. Module 2014 istypically attached to firmly adherent portions 1014b of conductors 1014because in some cases it is sufficiently heavy such that if attached toportions 1014a, it could endanger the integrity of portions 1014b ofconductors 1014 and cause same to crack and/or break.

In a typical embodiment, the module 2012 comprises an integrated circuitformed in a silicon chip of the flip chip or surface mount type,suitably covered by an insulating material (not shown) and having aplurality of external contacts 3012 (e.g., silver bumps) on the lowersurface 3412 thereof. These contacts 3012 are connected to electrodes(not shown) of various circuit elements (not shown) of the module 2012.The electrical module 2012 may take various other forms as previouslymentioned. It is to be noted that for illustrative purpose only, onemodule 2012 is shown. Typically, a printed circuit board assembly 1012contains a plurality of modules 2012 and 2014 of different types. Itmay, however, contain just one or more of modules 2012.

The conductive pattern formed by the conductors 1014 and the modules2012 and 2014 form an electrical circuit of the printed circuit boardassembly 1012. The conductors 1014 are typically formed by depositing apatterned layer of a suitable ink or paste that, after firing, providesa conductive pattern. Such conductors may include copper, nickel,silver, palladium, platinum and gold and/or alloys thereof and/or othersuitable metals and/or compounds to make them conductive.

In a preferred embodiment, the conductive layer was formed by the use ofDupont 9161, previously described, which comprises essentially finelydivided copper particles, a screening agent and adhesion promoters.

Each contact 3012 of module 2012 is typically connected to one part of aportion 1014a of a conductor 1014 by means of a bond, such as a solderjoint 3212. All of the solder joint bonds 3212 and 35 between bothmodules 2012 and 2014 and the conductors 1014 could be formed in variousother ways, including use of a conductive adhesive epoxy or otherconductive adhesive material, or could be welds.

In accordance with the invention, the various materials involved arechosen such that mechanical stresses on the solder joints 3212 aresignificantly relieved by the ability of the solder joints 3212 to moverelative to the circuit board 4012. The relative motion can be achievedin two basic fashions.

In the first, the portions 1014a of conductors 1014 remain intimatelybonded (attached) to portions of the inhibitor layer 1016 therebelow andtogether they move along and/or lift off circuit board 4012. Thisapproach has the potential of increasing the mechanical strength of thecomposite layer portions.

In the other approach, the portions of the inhibitor layer 1016 underportions 1014a of conductors 114 remain in place intimately bonded tothe circuit board 4012 while the portions 1014a of the conductors 1014are free to move along or lift off the portions of inhibitor layer 1016.

A conductive layer printed on a circuit board depends for its adhesionto the circuit board on a reaction between the surface of the board andthe ingredients in the composition (e.g., ink) used to print theconductive layer. For controlled adhesion of the kind desired, it isnecessary to partially stifle this reaction so that the layer canseparate from the circuit board in response to mechanical stress.

When the ink being used to print a conductive layer includes adhesionpromoters that react and insure firm adhesion where applied directly toa substrate, such as the surface of a printed circuit board, aninhibitor layer must serve to restrain such reaction.

There are several possible approaches to providing such an inhibitorlayer.

A first approach is to use an inhibitor layer that essentially includesonly an organic vehicle that has poor burn-out characteristics under thefiring conditions used, such as the case where a nitrogen atmosphere isused for firing a conductive layer (e.g., copper), so that littlereaction is created with the substrate for promoting adhesion becauseexcessive organics render the binders in the ink ineffective.

A second approach involves using as the inhibitor layer a film includingessentially only an organic vehicle that has excellent burn-out tolessen a chance of reaction between the adhesive promoters and thesubstrate. This approach is suitable in cases where no residual organicson the substrate are tolerable.

A third approach is to form the inhibitor layer by a composition thatconsists essentially of a finely divided refractory material (e.g.,alumina) suspended in an organic medium.

A fourth and presently preferred approach is to form the inhibitor layerby a composition that has a finely divided refractory material and aglass frit suspended in an organic medium. As mentioned earlier, theglass frit is chosen to soften and wet both the surface of the circuitboard and the refractory material, typically alumina, at a temperaturebelow the firing temperatures. This promotes bonding among therefractory particles and between the refractory particles and thesurface of the circuit board. The refractory material, preferably anoxide such as alumina, should have negligible reaction with theconductive layer to be deposited over the inhibitor layer.

In a specific embodiment of the invention in accordance with the lastapproach, the inhibitor layer 1016 was formed by printing a compositioncomprising by weight essentially about 80 percent a screening agent,about 10 percent a glass frit and about 10 percent alumina (Al₂ O₃).Moreover, the glass frit consisted by weight essentially of about 66.6percent lead oxide (PbO), 22.4 percent silicon dioxide (SiO₂), 8.7percent boron trioxide (B₂ O₃) and 2.3 percent alumina (Al₂ O₃).

Other suitable compositions for use in printing the inhibitor layer 1016included by weight: about 50 percent screening agent, 17 percent glassfrit, and 33 percent alumina; and about 40 percent screening agent,about 20 percent glass frit and about 40 percent alumina.

In these compositions, the screening agent used comprised by weightabout 25 percent of a resin, typically Elvacite 2046, a product of E. I.Dupont de Nemours Inc. dissolved in a solvent, such as TEXANOL™, aproduct that has been previously described, and the glass frit usedcomprised by weight between 60-75 percent PbO, 10-25 percent SiO₂, 5-30percent B₂ O₃ and 0-10 percent Al₂ O₃. Various glass frit compositionsother than the specific composition described can be used consistentwith the requirements mentioned above. Alternatively, certain oxides,such as bismuth oxide (Bi₂ O₃), which serve essentially the samefunction as the glass frit, can be substituted for the glass frit. Thesecertain oxides are denoted as "oxide binders".

The thickness of the inhibitor layer 1016 is desirably as thin as can beprinted reliably to minimize topographical changes in the conductors.The surface area of the inhibitor layer 1016 is sufficient to encompassthe regions desired for controlled adhesion but need not necessarily bepatterned to be congruent in width and length with the conductive layer.The thickness of the inhibitor layer 1016 is typically in the range of0.0001 to 0.001 inches with a preferred range of 0.0002 to 0.0005inches. A blanket inhibitor layer could be used instead of separatedinhibitor layers for each portion of the conductors which are to becharacterized by controlled adhesion. The thickness of the conductors1014 is typically in the range of 0.0003 to 0.001 inches with apreferred range of 0.0004 to 0.0008 inches. Generally, the thickness ofthe inhibitor layer after firing can be controlled both by the thicknessof the layer printed and the percentage of the inorganic solids (theglass fri and the refractory materials) in the composition printed.Generally, such solids should be between 5 and 85 percent by weight ofthe composition. The degree of adhesion is determined primarily by theratio of the amount of glass frit and the amount of refractory materialsin the composition. This ratio most advantageously is in the rangebetween 0.1 and 2.0.

The printed patterns can be formed in two basic ways.

The presently preferred way is first to print the composition thatprovides the inhibitor layer and merely to dry this layer withoutfiring. Then there is printed and dried the composition that providesthe conductive layer. The resultant is then heated for firing bothlayers in the same heating cycle. The heating cycle used is thatrecommended by the manufacturer for Dupont 9161 composition andbasically is that previously discussed with reference to FIG. 10.

Alternatively, depending on the nature of the selected conductor andinhibitor, the first layer could be both dried and fired beforedeposition of the second layer.

Examples of refractory oxides that can be used instead of Al₂ O₃ areSiO₂, ZrO₂ and TiO₂. A combination of any of these four refractoryoxides could also be used.

As mentioned previously, other refractory materials, such as nitrides,should be useful in place of the refractory oxides if compatible withthe glass frit.

Various other compositions for both the inhibitor layer and theconductive layer can be expected to be useful, consistent with theprinciples set forth including particularly various other screeningagents and amounts of screening agents

Moreover, the principles of controlled adhesion that have been discussedabove in the context of electrical application to printed circuit boardtechnology are not limited to such application. The principles can beextended to applications in which the controllably adherent conductorsserve primarily mechanical rather than primarily electrical roles. Theprinciples can further be extended to applications in whichnon-electrically conductive members (runners), which have essentiallythe same controlled adhesion as the controllably adherent conductors,are used in mechanical roles.

It is to be appreciated that the specific embodiments described aremerely illustrative of the general principles of the invention. Variousmodifications may be provided consistent with the principles set forth.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of forming aprinted circuit board assembly comprising the steps of:printing on acircuit board a pattern of a composition comprising by weight: a metalpowder as the major fraction; a screening agent comprising an organicvehicle including a solvent and a resin; and a binder comprising byweight between 0-70 percent bismuth oxide and the remainder is a fritthat includes by weight between 50-65 percent PbO, 5-15 percent PbF₂,20-30 percent SiO₂, 0-5 percent Al₂ O₃ and 5-15 percent B₂ O₃ and withthe proviso that said composition does not contain zinc oxide forpromoting controlled adhesion to the circuit board of a layer of thecomposition after drying and firing; drying and firing said compositionto form a controllably adherent conductor on the circuit board; andattaching at lest one electrical module to the controllably adherentconductors, and so that said conductor adheres to the circuit boardduring the step of attaching said electrical module but lifts off thecircuit board in response to stresses arising during operation and useof the printed circuit board assembly.
 2. The method of claim 1 therecomprising the step of printing on the circuit board firmly adherentconductors for interconnection with the controllably adherentconductors.
 3. The method of claim 1 there comprising the step ofprinting on the circuit board firmly adherent conductors forinterconnection with the controllably adherent conductors.
 4. A methodof forming a printed circuit board assembly comprising the stepsof:printing on a circuit board a pattern of a composition comprising byweight: a metal powder as the major fraction; an organic vehicleincluding a solvent and a resin as a smaller fraction; a binder as thesmallest fraction; the binder comprising by weight between 0-70 percentbismuth oxide and the remainder is a frit that includes by weightbetween 50-65 percent PbO, 5-15 percent PbF₂, 20-30 percent SiO₂, 0-5percent Al₂ O₃ and 5-15 percent B₂ O₃ and with the proviso that saidcomposition does not contain zinc oxide amount for promoting adhesion ofthe layer to the substrate during processing, cohesion of the metalpowder after firing, and controlled adhesion of the layer to the circuitboard whereby the layer is free to move along and/or lift off thecircuit board to relieve stresses; drying and firing said composition toform controllably adhering conductors on the circuit board; andattaching at least one electrical module to the controllably adherentconductors.
 5. A printed circuit board assembly comprising:a circuitboard having an insulative top surface; a patterned conductive layer onsaid top surface including firmly adherent portions and controllablyadherent portions; the controllably adherent portions having been formedfrom a composition comprising a metal powder as the major fraction, ascreening agent comprising an organic vehicle including a solvent and aresin, and a binder comprising by weight between 0-70 percent bismuthoxide and the remainder is a frit that includes by weight between 50-65percent PbO, 5-15 percent PbF₂, 20-30 percent SiO₂, 0-5 percent Al₂ O₃and 5-15 percent B₂ O₃ and with the proviso that said composition doesnto contain zinc oxide for promoting controlled adhesion to said topsurface after drying and firing; and an electrical module havingcontacts that are attached to the controllably adherent portions, and sothat said conductor lifts along the circuit board in response tostresses arising during operation and use of the printed circuit board.6. The printed circuit board claim 5 in which the metal powder is copperof an average particle size between 1 and 2 microns and in which thebinder consists essentially of bismuth oxide.
 7. The printed circuitboard assembly of claim 5 in which the metal powder is copper of anaverage particle size between 1 and 2 microns and in which the bindercomprises a mixture consisting essentially of bismuth oxide and cuprousoxide.
 8. A printed circuit board assembly comprising:a circuit boardhaving an insulative top surface; a patterned conductive layer on saidtop surface including firmly adherent portions and controllably adherentportions; the controllably adherent portions having been formed from acomposition comprising a metal powder as the major fraction, a screeningagent comprising an organic vehicle including a solvent and a resin, anda binder of composition and amount for promoting controlled adhesion tosaid top surface after drying and firing; an electrical module havingcontacts that are attached to the controllably adherent portions andsaid binder consisting of Bi₂ O₃, PbO, PbF₂, SiO₂, Al₂ O₃ and B₂ O₃. 9.A printed circuit board assembly comprising:a circuit board having aninsulative top surface; a patterned conductive layer on said top surfaceincluding firmly adherent portions and controllably adherent portions;the controllably adherent portions having been formed from a compositioncomprising a metal powder as the major fraction, a screening agentcomprising an organic vehicle including a solvent and a resin, and abinder of composition and amount for promoting controlled adhesion tosaid top surface after drying and firing; an electrical module havingcontacts that are attached to the controllably adherent portions, andwherein the binder comprises by weight between 0-70 percent bismuthoxide and the remainder is a frit that includes by weight between 50-65percent PbO, 5-15 percent PbF₂, 20-30 percent SiO₂, 0-5 percent Al₂ O₃and 5-15 percent B₂ O₃ and with the proviso that said composition doesnot contain zinc oxide.
 10. A printed circuit board assemblycomprising:a circuit board having a patterned conductive layer on thetop surface of the circuit board including a first portion firmlyadhered to the circuit board and a second portion; the second portionhaving been formed from a composition comprising a metal powder; ascreening agent, and a binder consisting essentially of Bi₂ O₃, PbO,PbF₂, SiO₂, Al₂ O₃ and B₂ O₃ in an amount sufficient after drying andfiring the layer to promote movement of the layer along the circuitboard in response to stresses; and an electrical module having contactsthat are attached to the second portion of said conductive layer.